【误】 She was on the point of leaving while someone knocked at the door.

【正】 She was on the point of leaving when someone knocked at the door.

【析】 while意为"当......时候"，从句需用持续性动词或状态性动词；而when引导时间状语从句中的动词既可以是持续性动词也可是非持续性动词。

误区之二：错用till / until

【误】 Until / Till yesterday evening, I received her

e-mail from Shanghai.

【正】 I didn't receive her e-mail from Shanghai until yesterday evening.

【析】 当until / till从句与肯定的主句连用时，主句谓语动词必须是持续性动词，而不可是非持续性动词。

误区之三：省略不当

【误】 While was walking down the street I noticed a police car in front of number 88.

【正】 While walking down the street I noticed a police car in front of number 88.

【析】 时间、条件、让步等状语从句中，如果从句主语与主句主语一致或是it，且从句中含有动词be的某种形式，从句主语和be可以同时省略。

误区之四：倒装语序误用

【误】 Not until the early years of the 19th century man did know what heat is.

【正】 Not until the early years of the 19th century did man know what heat is.

【析】 Not until位于句首时，主句谓语采用倒装语序。

误区之五：从句时态错用

【误】 I won't know where she is until I will get a letter from her.

【正】 I won't know where she is until I get a letter from her.

【析】 在时间或条件等状语从句中，用现在时表示将来时。

误区之六：错用however

【误】 However he is late, Mother will wait for him to have dinner together.

【正】 However late he is, Mother will wait for him to have dinner together.

【析】 however需和它修饰的词一起放在从句前引导状语从句。

误区之七：混用though与although